Pressurized water nuclear reactors comprise a vessel of generally cylindrical shape which is placed, in the service position, with its axis vertical, and which comprises a closing cover in its upper part and a dome-shaped bottom in its lower part.
Extending through the cover of the vessel are adapters in the form of vertically disposed tubes which allow the passage of extensions of control clusters controlling the reactivity of the core disposed inside the vessel, or of thermocouple columns whereby it is possible to measure the temperature in different regions of the core of the reactor during its operation.
The adapters comprise a portion which projects above the cover of the vessel and may be of great height, the upper end of which, in the case of adapters allowing the passage of extensions of control clusters, carries a mechanism controlling the displacement of an extension and a control bar.
Extending through the dome-shaped bottom of the vessel are a plurality of substantially vertical tubes termed vessel bottom penetrations.
These vessel bottom penetrations comprise an end portion which projects below the dome-shaped bottom and is connected to a flexible measuring conduit whereby it is possible to connect the bottom of the vessel to an instrumentation room disposed in the building structure of the reactor.
Each of the measuring conduits and the corresponding vessel bottom penetration allows the passage of a thimble in which is displaced a measuring probe fixed to the end of a flexible element of great length and the introduction into the interior of the vessel and of the core of the measuring probe for effecting measurements, for example of the neutron flux or the temperature within the core during operation of the reactor.
The through adapters of the cover of the vessel and the vessel bottom penetrations are usually composed of nickel alloy and are fixed by welding to the cover or to the dome-shaped bottom of the vessel, which are made of a steel of high strength.
The through tubes, whether adapters or vessel bottom penetrations, are engaged in cylindrical through openings produced by drilling the cover or the dome-shaped bottom of the vessel.
Generally, the through openings have a diameter slightly less than the nominal diameter of the through tubes at the ambient temperature, and the tubes are mounted after having brought them to a low temperature, for example to the temperature of liquid nitrogen. In this way, the tube is gripped by a shrink-fit in the dome-shaped bottom.
The fluidtight fixing of the through tube in the dome-shaped wall is completed by a weld produced on the concave surface side of the dome-shaped bottom around the end of the tube, which projects from or is substantially flush with this concave surface.
Generally, the weld is effected by deposit of a filler metal in an annular bevel machined in the dome-shaped wall in the region of its concave surface around the through opening for the tube.
In the case of tubes of nickel alloy, the bottom of the annular welding bevel is covered prior to welding with a buttering coating of nickel alloy similar to the alloy of the through tube. The filler metal deposited in the welding bevel itself has a composition similar to or very close to the composition of the nickel alloy of the through tube.
The welding is generally effected by an arc welding process and requires many successive passes in order to fill the bevel with the filler metal.
The process for mounting and welding a through tube of a dome-shaped wall of a nuclear reactor vessel which requires many successive operations is consequently complex, long and costly. Owing to the fact that the weld is produced by deposit of a relatively large amount of liquid metal in the bevel, the shrinkage of the metal upon solidification creates considerable local deformations in the welded region and a region of high residual stresses. If the metal of the tube is subject to corrosion under the effect of tension, a crack may appear in service. Consequently, after a certain period of operation of the nuclear reactor, the through tubes may show cracks produced by corrosion under tension, in particular in the region of the weld between the through tube and the dome-shaped wall.
It may then be necessary to repair or replace the defective through tube.
In the case where the defective tube is replaced, the operations for positioning and welding the tube may be relatively long and costly, even if the weld produced by the deposit of filler metal is effected inside a narrow bevel of small volume machined in the metal of the weld of the tube being replaced.
In the case of through adapters of vessel covers, the complete replacement of the cover has been envisaged in the case where adapters showed cracks, so as to ensure perfect safety after changing the cover and again starting up the nuclear reactor. In this case, it may be necessary to have very rapidly available a new vessel cover in order to reduce as far as possible the shut-down time of the nuclear reactor.
Owing to the fact that the vessel cover is a very expensive component, it is preferable to manufacture replacement vessel covers on order and for a precise utilization.
It is therefore desirable to reduce as far as possible the time it takes to manufacture vessel covers.
More generally, it is of course also necessary to reduce as far as possible the time it takes to manufacture the whole of the vessel of a nuclear reactor so as to limit manufacturing costs.
Among the operations necessary for manufacturing a nuclear reactor vessel which have a tendency to lengthen the manufacturing time, the fluidtight fixing of the through tubes of the cover and of the bottom of the vessel is particularly important, owing to the large number of through tubes.
It is therefore quite desirable to reduce as far as possible the time required to fix in a fluidtight manner the adapters and the vessel bottom penetrations.
It is also desirable to have available a process for rapidly and inexpensively replacing defective adapters or vessel bottom penetrations of a nuclear reactor, following a certain period of operation.